Here's what you need to motorize your mill all from Grainger:
V Belt, Nominal Outside Length 44 Inches, Top Width 3/8 Inch, Belt 3L, 7/32 Inch Thick, Smooth DAYTON
3L440-G
Die Cast One Groove Sheave, OD 1 1/2 Inches, 3L Belt Pitch Diameter 1.05 Inches, 4L or A Belt Pitch Diameter 1.35 Inches, 1/2 Inch Bore Size CONGRESS
CA150 1-1/2A X 1/2
Die Cast One Groove Sheave, OD 10 Inches, 3L Belt Pitch Diameter 9.55 Inches, 4L or A Belt Pitch Diameter 9.85 Inches, 1/2 Inch Bore Size CONGRESS
CA1000 10A X 1/2
Belt Drive Motor, Split-Phase, Open Dripproof, 1/3 HP, 1725 RPM, Number of Speeds 1, 115 Volts, 6.6 Full Load Amps, 48Y NEMA Frame, Service Factor 1.35 DAYTON
6K778G
Adapter Bushing, 3/8 Inch ID x 1/2 Inch OD, 1 1/8 Inches Length, For Adapting 3/8 to 1/2 Inch Diameter Shaft DAYTON
6X452
This is what I used to make mine:
